1. Field of the Invention
The present invention relates to an output control system for an internal combustion engine.
2. Description of the Related Art
In order to lower an output torque of an internal combustion engine for controlling an engine output, there is a method to placing a part of or all of engine cylinders resting by performing fuel cut-off (F/C) or ignition cut-off.
In this case, in order to control the lowering magnitude of the engine output torque in stepwise fashion, the number of the engine cylinders placed in resting state is to be controlled.
As such a control system, a fixed cylinder resting type engine output control system has been known in the technologies disclosed in Japanese Unexamined Patent Publications Nos. SHO 58-8436 and HEI 1-130018.
In this case, the cylinders to be placed in the resting state is preliminarily determined for respective modes depending upon the lowering magnitude of the engine output torque. For instance, in the case of 4-cylinder engine, a map illustrated in FIG. 13 can be used. In FIG. 13, #1, #2, #3 and #4 represent the 1st cylinder to 4th cylinders. Also, .smallcircle. represent fuel supply timing and x represent fuel cut-off timing, i.e. F/C. It should be appreciated that .smallcircle. and x will be used in the same meanings in the following description.
However, the above-mentioned engine output control system is not satisfactory in a response to initiation of the control.
For instance, as shown in FIG. 14(A), when a command for mode 1, in which F/C is to be effected for one cylinder and when the combustion stroke of the cylinder to be effected the F/C is scheduled at a timing immediately after occurrence of the command, the F/C for #1 cylinder can be effected. However, as shown in FIG. 14(B), if the combustion timing of the cylinder to be effected the F/C is immediately before the occurrence of the command (in other words, when the cylinder combustioned immediately before is commanded as the cylinder to be rested immediately after combustion), the period (corresponding to two turns of rotation of the crankshaft, i.e. one engine cycle) up to the next occurrence of combustion becomes wasted. In such case, the response characteristics in the engine output control becomes bad.